1. Field of the Invention
The present invention relates to limited slip differential assemblies for motor vehicles, and more particularly to a limited slip differential assembly having an electronically controlled hydraulic actuator including an electro-magnet actuated variable pressure relief valve for limiting a relative rotational speed between two driven shafts above a predetermined amount.
2. Description of the Prior Art
Conventionally, differentials well known in the prior art, are arranged in a power transmission system of a motor vehicle to allow a pair of output shafts operatively coupled to an input shaft to rotate at different speeds, thereby allowing the wheel associated with each output shaft to maintain traction with the road while the vehicle is turning. Such a device essentially distributes the torque provided by the input shaft between the output shafts. However, these types of differentials known in the art as an open differentials, i.e. a differential without clutches or springs, are unsuitable in slippery conditions where one wheel experiences a much lower coefficient of friction than the other wheel; for instance, when one wheel of a vehicle is located on a patch of ice or mud and the other wheel is on dry pavement. In such a condition, the wheel experiencing the lower coefficient of friction loses traction and a small amount of torque to that wheel will cause a xe2x80x9cspin outxe2x80x9d of that wheel. Since the maximum amount of torque, which can be developed on the wheel with traction, is equal to torque on the wheel without traction, i.e. the slipping wheel, the engine is unable to develop any torque and the wheel with traction is unable to rotate. Thus, the necessity for a differential, which limits the differential rotation between the output shafts to provide traction on slippery surfaces, is well known.
Such differential assemblies are typically called limited slip differentials. Conventionally, they use a frictional clutch between the side gear and the differential case. The frictional clutch may be selectively actuated by various hydraulic actuator assemblies, which are constructed of elements disposed inside the differential casing. The hydraulic actuator assemblies internal to the differential case often include displacement pumps disposed inside the differential casing and actuated in response to a relative rotation between the differential case and the output shaft. The displacement pumps are usually in the form of internal gear pumps, such as gerotor pumps adapted to convert rotational work to hydraulic work. In the internal gear pumps, an inner gear having outwardly directed teeth cooperates with an external gear having inwardly directed teeth so that fluid chambers therebetween increase and decrease in volume as the inner and outer gears rotate in a housing. By connecting the inlet and outlet of the device to the proper location along the sides of the gear set, the variable displacement chambers receive and discharge hydraulic fluid so that the device can function as a pump or motor. A shaft or other mechanical device can be connected to either the inner or outer gear depending upon the type of device. The hydraulic actuator assemblies further include a hydraulic piston member for frictionally loading the friction clutch.
Recent advances in vehicle control may require the disabling of the limited slip feature of the differential at moderate to high speeds. One such system is the yaw stability control, which uses the vehicle""s brakes to correct the trajectory of the vehicle during a turn. The impulse braking of the yaw stability control feature generates a speed difference between the wheels on either side of the vehicle. The limited slip feature will engage due to this speed difference and may interfere with the performance of the yaw stability control feature. There is therefore a need to disable the limited slip feature of the hydraulic limited slip differential during specified conditions to ensure proper performance of the devices like yaw stability control while also allowing the limited slip feature to be enabled at other specified conditions where traction may be needed and where yaw control is not essential. There is a problem with current hydraulically actuated limited slip differentials in that they do not have a simple on/off capability which is separate and distinct from the hydraulic pressure supply/control circuit actuating the clutch assemblies.
It is the intent of this invention to overcome these shortcomings by providing an external control of the hydraulic pressure generated within a hydraulically actuated limited slip differential in which the limited slip clutch can either be turned on or off, or set at any intermediate condition by controlling the maximum system hydraulic pressure limit.
The present invention provides an improved electronically controlled differential assembly providing both limited slip and open differential capabilities.
The differential assembly in accordance with the preferred embodiment of the present invention includes a rotatable differential case rotatably supported within an axle housing and forming housing a differential gearing rotatably supported in the case and a pair of opposite output axle shafts in meshing engagement with the differential gearing to permit differential rotation thereof. The differential assembly includes a friction disk clutch pack disposed within the differential case and provided to selectively lock the differential assembly. The friction clutch pack includes a number of alternating outer friction plates non-rotatably coupled to the differential case and inner friction plates drivingly coupled to one of the output axle shafts. The differential assembly further includes a hydraulic clutch actuator for selectively frictionally loading the clutch pack. The hydraulic clutch actuator comprises a hydraulic pump for generating a hydraulic pressure, a piston assembly disposed within the differential case between the pump and the clutch pack and defining a pressure chamber, and a variable pressure relief valve assembly to selectively control said limited slip assembly. The variable pressure relief valve assembly has a pressure relief valve and a solenoid actuator for selectively setting a maximum hydraulic pressure attainable within the pressure chamber between a maximum release pressure when the friction clutch pack is in the fully xe2x80x9cONxe2x80x9d condition, and a minimum release pressure when the friction clutch pack is in the fully xe2x80x9cOFFxe2x80x9d condition.
The variable pressure relief valve assembly includes a valve closure member, a valve seat complementary to the valve closure member, and an electro-magnetic actuator for engaging the valve closure member and urging thereof against the valve seat with an axial force determined by a magnitude of an electric current supplied to the electro-magnetic actuator so as to selectively vary a release pressure of the pressure relief valve assembly based on the magnitude of the electric current.
Therefore, the differential assembly in accordance with the present invention allows to dynamically control the hydraulic pressure of the hydraulic clutch actuator for selectively frictionally loading the clutch pack, thus providing an optimized amount of limited slip between the fully xe2x80x9cONxe2x80x9d and xe2x80x9cOFFxe2x80x9d conditions.